In general, an engine is the power generating mechanism that provides energy for a vehicle and principally includes a lubricating device, a fuel device, a cooling device, a suction/exhaust device, an ignition device, and the like.
An engine is a heat engine, which uses fuel as a heat source and supplies heat energy generated by the fuel to an operating fluid to burn and explode the fuel. The engine transforms the force generated by the combustion of the operating fluid to a rotational force on a shaft, thereby creating mechanical energy for obtaining power.
In other words, heat energy is transformed to the reciprocating movement of a piston, and the reciprocating movement is again transformed to a rotational power.
Conventionally, in order for a gas engine to continue the periodic movement of a piston via a burning process, burnt gas should be discharged and a new fuel-air mixture should be supplemented. As a result, suction/exhaust valves should be opened and closed in an accurate manner in coordination with the rotation of a crankshaft.
The periodic change of a piston in a cylinder is called an engine cycle, where mixed gas is sucked into a cylinder to be burnt and an exhaust gas generated therefrom is discharged outside. A gasoline engine typically has two types of cycles, one being a 4-cycle engine and the other being a 2-cycle engine. But a 4-cycle engine is mainly used for wide ranges of speed change and good fuel consumption.
In particular, a 4-cycle engine is an engine that completes a fuel-burning process in four strokes, where one stroke means a movement or a distance from an upper dead center to a bottom dead center of a piston.
During an intake stroke, the piston descends from the upper dead center to the bottom dead center sucking the fuel-air mixture, during which the suction valve is opened while the exhaust valve is closed.
A compression stroke is a stroke in which the piston ascends from the bottom dead center to the upper dead center to compress the sucked fuel-air mixture and during which the suction valve and the exhaust valve are closed such that the pressure and temperature of the fuel-air mixture increase. Accordingly, a piston carries out two strokes to rotate a crankshaft one time.
An expansion stroke is a stroke in which the compressed fuel-air mixture is ignited so that the mixture expands, and pressure generated therefrom presses down on the piston to generate actual power during which the suction valve is closed. Gas expands in response to the descent of the piston, while the pressure and temperature decrease.
An exhaust stroke is a stroke in which a piston at the bottom dead center is again moved to the upper dead center to discharge the expanded gas outside of the piston during which only the exhaust valve is opened.
When a piston completes the 4-stroke cycle that includes the suction, compression, expansion and exhaust strokes, the crankshaft is rotated twice to finish one operation of a power generating movement.
However, the size of rotational force, in relation to changes in the inertia force of the moving parts and combustion pressure inside a piston, is increased and decreased repeatedly in response to the rotational angle of a crankshaft. In other words, the rotational force of the crankshaft reaches its highest point during the expansion stroke of the first cylinder and is gradually decreased and again reaches the highest point during the expansion strokes of a third cylinder, a fourth cylinder, and a second cylinder. These movements are repeated to resemble a sine curve. The big fluctuation in rotational force as the crankshaft periodically changes its rotation speed such that a flywheel is typically mounted at a rear end part of the crankshaft in order to obtain a predetermined size of rotational force.